Intervertebral disks provide mobility and a shock cushion between the vertebrae. Anatomically, a disk includes the nucleus pulposus in the center, which is composed of cells and an insoluble extracellular matrix which is produced by the nucleus itself. The extracellular matrix is composed of collagen, proteoglycans, and water. The nucleus pulposus is surrounded by the annulus fibrosis, which is also composed of cells (fibrocyte and chondrocyte), collagen fibers, and an extracellular matrix. The endplates are attached to the bony tissue, the vertebrae.
To date, there are a few treatments for degenerative disk disease. Spinal fusion has been widely used to obtain mechanical support and stability in various clinical situations, such as degenerative disk disease or infective and malignant conditions that induce spinal instability. Although results are satisfactory in most cases, long term complications, such as adjacent-segment degeneration, have been reported and remain an issue. The mechanism of this degeneration is still unclear. Cadaver studies demonstrated increasing intradisk pressures in the adjacent unfused intervertebral disks after a simulated spinal fusion. Retrospective clinical analysis suggested that the risk of adjacent-segment failure is higher for patients in whom lumbar fusion with rigid instrumentation is performed. A few studies indicate that the altered mechanical environment after short rigid spinal fusion lead to the accelerated adjacent segment degeneration.
To avoid altering the biomechanical function of the intervertebral joint, efforts have been made to develop an artificial disk, as reported a few issued patents (U.S. Pat. Nos. 6,958,078, 6,989,032, and 6,692,495 (all incorporated by reference herein)). Some of the difficulties with such developments include implant fixation, biocompatibility, and the identification of suitable materials and designs, which duplicate both form and function. Clinically, devices that replace a total disk exhibit certain weaknesses. Total disk replacements rely on attachments of the artificial disk to the vertebral endplates. Various methods of attachment have been described including the use of screws, spikes, and porous ingrowth material. The total disk/vertebral interface can loosen due to non-natural biomaterials. The problems with prosthesis loosening and revision surgery of the same are reported to be similar to that encountered with total knee and hip replacement surgery. As such, the long term outcomes of total replacement by artificial disks are still under review.
The desired treatment for degenerative disk disease therefore lies in treatment which preserves natural disk function. If disk function could be restored with biologic natural disk replacement, the weaknesses of current artificial disk replacement would be minimized, if not eliminated.